To set a thickness of a molded multilayered film/sheet to be constant during molding of the multilayered film/sheet by a multi manifold die (T-die), a flow rate distribution of a molten resin needs to be uniform in a width direction of the die after molten resins of layers are combined. Moreover, needless to say, a die dwell time distribution needs to be uniform in the width direction of the die. Furthermore, the flow rate distribution of the molten resins of the respective layers before the resins are combined needs to be also uniform in the width direction of the die, and the die dwell time distribution needs to be also uniform in the width direction of the die.
In addition, to obtain a uniform flow rate distribution and a uniform die dwell time distribution, in the die for molding the multilayered film/sheet by a multi manifold method, a shape of each-layer manifold or a pressure loss adjustment land is determined by simulation and analysis by a computer. In this case, the shape of the manifold or the pressure loss adjustment land is a considerably complicated curved shape, but working of the complicated curved shape has become possible by the use of a high-precision numerical control machine tool.
In the molding of the multilayered film/sheet, the characteristics of resin change for each layer depending on operation conditions of a film/sheet molder or a resin type same as in molding of a single-layer film/sheet. The resin characteristics vary depending on the operation conditions of the film/sheet molder. Therefore, even when the shapes of the manifold and the pressure loss adjustment land are optimized as described above, it is difficult to obtain a uniform thickness of the film/sheet on all operation conditions.
Moreover, the molten resins of the respective layers are combined in the die, and the junction is influenced by a difference in the resin characteristics of the layers or characteristics of a target-layer resin. It is difficult to completely simulate a resin behavior in this junction, and the behavior is often simulated for a simple model.
Heretofore, the thickness of the multilayered film/sheet fluctuates with the operation conditions, the resin type, and further the combination of the respective layers. The fluctuation is compensated for by changing a gap between choker portions for each layer to thereby adjust the thickness of each layer. A gap between lips in an outlet of the die is adjusted to thereby adjust the thicknesses of all the layers.
A die for molding a two-layer film/sheet is often provided with a choker portions in molten resin channels of the respective layers, respectively. In a die for molding a three-layer film/sheet, when all of the three layers are provided with a choker portion, a die structure becomes complicated. Moreover, the die shape becomes very large, and it is difficult to handle the die easily. Therefore, in general, the choker portion is disposed only in the outer-layer or inner-layer molten resin channel in many cases.
However, in a case where the choker portion is disposed only in the outer-layer or inner-layer molten resin channel, it is difficult to precisely adjust each layer. Therefore, it is difficult to mold a film/sheet which has a satisfactory precision of thickness.
The gap between the choker portions is adjusted by turning a choker adjustment bolt, and moving a choker bar in a vertical direction (a direction to change a sectional area of the molten resin channel in the die). In an ordinary die, a plurality of choker adjustment bolts are arranged at a pitch of about 30 to 60 mm in the die width direction.
To set the thickness of the film/sheet to be uniform, these choker adjustment bolts are operated, and the gap between the choker portions is adjusted every portion provided with each choker adjustment bolt. In general, the choker bar has a square bar configuration, and is made of the same type of steel material as that of a die main body.
In the choker portion gap adjustment which is performed by turning each choker adjustment bolt to change a movement of the choker bar, the choker bar is bent every pitch of several choker adjustment bolts, and the gap between the choker portions is changed depending on the shape of the bent bar. The choker bar cannot be bent into a wavy shape at every choker adjustment bolt. That is, it is difficult to adjust a small gap between the choker portions at a small pitch, and the thickness cannot be adjusted at a small pitch.
An automatic control system is known in which a rotary actuator is attached to each choker adjustment bolt, the thickness of the film/sheet is automatically measured, each rotary actuator is feedback-controlled by the measured value, and the thickness of the film/sheet is automatically controlled.
In this case, the gap between the choker portions is automatically adjusted, but the small gap between the choker portions cannot be adjusted at the small pitch, and the thickness adjustment at the small pitch cannot be performed in the same manner as in the manual adjustment.
If a choker portion is provided for every of the three layers, as described above, the die structure becomes complicated, and large in scale. Therefore, the choker portion is disposed in the only outer-layer or inner-layer molten resin channel in many cases.
Moreover, there is also known a die for molding a multilayered film/sheet, in which a cartridge heater is buried in the die main body, the main body is heated with a heater to change the viscous characteristics of the molten resins in the molten resin channels of the respective layers, and a flow rate of the molten resin is controlled to thereby adjust the thickness of the film/sheet.
There is also a system in which a plurality of cartridge heaters are arranged at a predetermined pitch from the back of the die in a die width direction, and the temperature of each heater is adjusted to thereby change flow characteristics of the resin in the manifold and adjust the thickness of the film/sheet. In this system, when the molten resin flowing from the center of the die into the manifold spreads in the width direction of the die in the manifold, the molten resin flowing toward die end portions is influenced by the heater at the center of the die.
Therefore, during the adjustment of the thickness of the film/sheet, complicated control is required for the die end portion in consideration of the influence of upstream control at the center of the die. From viewpoints of resin characteristics, it is sometimes difficult to set the thickness of the film/sheet to be constant due to mutual interference with the temperature controls on upstream and downstream sides.
In a system in which a plurality of cartridge heaters are arranged at a predetermined pitch in a die outlet width direction, and the temperature of each heater is adjusted, it is possible to individually control the heaters in the width direction. Therefore, the system is effectively used in molding a film or a thin sheet. However, in case of a thick sheet, it is difficult to sufficiently control the thickness of the sheet due to a lack of quantity of heat of the heater, and therefore the system is hardly used.
In the die for molding the three-layer film/sheet, the cartridge heater disposed between the outer-layer molten resin channel and the inner-layer molten resin channel heats the outer-layer and inner-layer molten resin channels at the same time, and it is difficult to precisely control the temperature of the molten resin of each layer. It is considered that an insulating portion made of an insulating material be disposed between this heater and the outer-layer molten resin channel so as to prevent the outer-layer molten resin from being unnecessarily influenced by this heater, but it is often difficult to dispose the insulating portion in a limited structure space.